1. Technical Field of the Invention
This invention relates generally to a drive train clutch mechanism for a motor vehicles, and more particularly to a motor vehicle having a both a gearbox clutch which performs the conventional coupling and decoupling of the engine from the driven wheels, and a separate slipper clutch which limits back-torque from the driven wheels to the engine such as under deceleration.
2. Background Art
FIG. 1 illustrates an engine 10, which may be an internal combustion engine or, for example, an electric motor. The engine includes a crankcase 12 which encloses one or more crankshafts 13, 15 as well as a gearbox or transmission 17. A cylinder block 14 is coupled to the crankcase, and a head 16 is coupled to the cylinder block. The engine is illustrated as an inline four-cylinder four-stroke engine, may be any suitable engine or motor configuration. In the case of an electric motor, the drive shaft may be considered a crankshaft, and the motor housing may be considered a crankcase.
The engine includes a primary or gearbox clutch 20 which serves to selectively couple and decouple the crankshaft or other drive train component from the final drive output 21 of the engine. In a conventional, transverse-crank motorcycle engine, the gearbox clutch is typically located on a side of the engine. The gearbox clutch typically includes a stack (not shown) of alternating drive plates and friction plates, in which the drive plates interlock with a clutch basket 23 and the stack is kept under compression by a set of clutch basket springs (not shown). A cable or hydraulic actuator mechanism (not shown) relieves the spring compression, allowing the various plates in the stack to rotate and slip on one another, decoupling the crankshaft from the final drive output.
When the engine is throttled up for acceleration, if the clutch is not slipping, the force of the crankshaft is coupled through to the final drive, accelerating the vehicle's driven wheels. However, when the engine is throttled down for deceleration, if the clutch is not slipping, the reduced force on the crankshaft is transmitted through the clutch to the final drive, and the vehicle experiences “engine braking”. Engine braking can be a significant force, and not only causes stresses in the drive train components (in a direction opposite to that for which they may have been engineered), but may also reduce the stability of the vehicle or have other negative consequences. This is a well-known problem in certain applications, especially high-compression, large displacement four-stroke racing motorcycles.
In racing applications, it is often desirable to limit the effects of engine braking, to reduce engine stress and to improve rideability and control. Many motorcycles are equipped with back-torque-limiting “slipper clutches” which include a sprag bearing which locks up in one rotational direction but freewheels in the other direction. It is generally not desirable to entirely eliminate engine braking. Most slipper clutches have, in addition to their sprag, a controlled and sometimes statically adjustable amount of clutch friction applied in the freewheel direction, to provide some amount of engine braking.
Unfortunately, the gearbox clutch is subjected to very high stress and temperature during vigorous operation, such as in racing conditions. The stress and temperature degrade the performance and predictability of the slipper function. In the 2002 MotoGP racing season, several riders, including Valentino Rossi and Kenny Roberts Jr., were known to have achieved reduced results in certain races specifically because of slipper clutch problems. Clutches are subject to maximum abuse at the starting line, followed by continued hard usage throughout the race. A racer's mechanics may achieve a perfect pre-race adjustment of a rider's slipper clutch, then, over the course of the race, as the clutch wears and is repeatedly heated and cooled, the slipper clutch falls farther and farther out of adjustment. This may have a significant impact on a rider's braking points and confidence.
What is needed, then, is an engine having an improved slipper clutch function, in which the slipper clutch is less affected by the condition of the gearbox clutch function.